The present invention generally relates to laser sources and further to equipments for measuring optical components.
In optical communication networks, information is generally transmitted by optical fibers from a stimulus, e.g. a laser diode, to an optical receiver, e.g. a photo diode. These may not only be point-to-point links but may also provide a complex network structure which generates the need for optical components for data routing, adding, dropping and switching.
To increase the transmission capacity, several communication channels are normally used simultaneously. In principle, this can be realized by separating the channels by providing time multiplexing or centering channels at different wavelengths. The latter principle is also known as Wavelength-Division-Multiplexing (WDM) and is becoming increasingly important. A state of the art WDM system has about 20 channels separated by 0.8 nm in a wavelength range of around 1550 nm. First research work is already done to increase the amount of channels by reducing the channel spacing down to 0.2 nm and therefore increase the transmission capacity by about four times.
One of the problems by using WDM is the interference (cross talk) of the communication channels. To avoid interference, the used optical components need to exhibit a high wavelength dependent transmission characteristics, that is, e.g., a WDM cross-connect switch with a transmission dynamic of up to 30 dB over tenths of a nanometer. A known complex and expensive measurement setup for characterizing this kind of optical components is based on a tunable laser source, a wavelength meter, a tracking filter and an optical power meter (cf., e.g., in AFiber optic test and measurement@ by Dennis Derickson, ISBN 0-13-53480-5, page 358 ff.).
In general, the signal to total noise ratio of a laser source (e.g. a tunable laser source as depicted in the above mentioned book by Dennis Derickson on page 360) limits its applications where high transmission dynamics characteristics have to be measured, e.g., in case of a notch-filter with a high signal suppression, such as a fiber grating, where the back noise (SSE, ASE) of the laser source determines the measured suppression of a signal positioned at a center wavelength of the filter (cf. FIG. 6).
A solution to improve the signal-to-noise ratio of a laser system is to provide a filter in combination with a broadband receiver or an optical spectrum analyzer. To ensure also the wavelength accuracy of the measurement which is very important in WDM systems with narrow channel spacing, also known as Dense Wavelength Division Multiplexing (DWDM), an external wavelength meter has to be used. For all these setups an additional controller plus software is needed for synchronizing and data capturing.
JP-A-06 140717, Lewis L. L. in xe2x80x9cLow noise laser for optically pumped cesium standardsxe2x80x9d (proceedings of the annual frequency control symposium, Denver, May 31-Jun. 2, 1989, no. Symp. 43, May 31, 1989, Institute of electrical and electronics engineers, pages 151-157, XP000089353), and Boshier M. G. et al. in xe2x80x9cExternal-Cavity frequency stabilization of visible and infrared semiconductor lasers for high resolution spectroscopyxe2x80x9d (Optics communications, vol. 85, no. 4, Sept. 15, 1991, pages 355-359, XP000226852) disclose laser systems with a beam splitter provided in the external cavity. FIG. 1 shows in principle a laser source 5 according to those art documents.
A laser gain medium or amplifier 10 provides a first facet 20 which is low reflective and a second facet 30 which is high reflective. The first facet 20 emits a laser beam 50 into an external cavity of the laser source 5. A collimating lens 60 collimates the laser beam 50 to a beam splitter 65 splitting the laser beam 50 into a part 50xe2x80x2 and a part 67. The part 50xe2x80x2 of the laser beam 50 is directed to an optical grating 70 as a wavelength dependent mirror. The optical grating 70xe2x80x3 diffracts the incident beam 50xe2x80x2 and a wavelength separated beam 50xe2x80x3 is directed back towards the beam splitter 65. The angle of the optical grating 70 with respect to the beam 50xe2x80x3 depends on the wavelength to be selected. The optical grating 70 together with the facet 30 of the semiconductor amplifier 10 define the optical resonator of the laser source 5. The beam splitter 65 splits up the returning beam 50xe2x80x3 into a beam 50xe2x80x2xe2x80x3 towards the gain medium 10 and a beam 80. The laser system 5 provides as output signals the laser beams 67 and 80, coupled out respectively from the beam splitter 65. The output beam 80 can be coupled into a fiber 90, e.g., by means of an optical lens 100.
Jeffrey Bernstein et al. in xe2x80x9cOscillator design improves dye-laser performancexe2x80x9d (Laser Focus World, September 1995, pages 117 ff.) discloses that the laser beam 80, which is substantially coupled out directly after wavelength selection by the optical grating 70 provides an improved lower signal-to-noise ratio output with respect to the output beam 67.
In modem laser applications, in particular for measuring purposes e.g. for measuring modem optical components for DWDM, it becomes increasingly important to provide flexible laser systems offering a wide range of laser signals from high power signals to low noise signals. Although the output.67, coupled out directly the gain medium 10 in the laser system 5 of FIG. 1, provides a possibility for a higher power output with regard to the output 80, the output 67 finds a power limitation in the beam splitter 65. Since the beam splitter 65 couples out as well the beam 80 as the beam 67 necessarily with the same coupling-out-ratio, or in other words, since the beam splitter 65 couples out the power of beam 67 or 80, a certain tradeoff between the possible power to be coupled out and the resonator conditions for an efficient power amplification by the external cavity has to be found. This, however, limits the possible applications of the laser systems and generally renders them to be not sufficiently flexible.
It is thus an object of the present invention to provide a flexible laser system offering a wide range of laser signals from in particular high power signals to low noise signals. It is a further object to provide a low cost and smaller flexible laser system. This object is solved by independent claims 1 or 2. It is another object to provide a low cost measuring setup for determining high transmission dynamics characteristics. This object is solved by independent claim 8. Preferred embodiments are shown by the dependent claims.
The invention is based on a laser source with an optical resonator. The laser source comprises a laser gain medium, e.g. a semiconductor and/or fiber amplifier, for emitting a laser beam, a wavelength dependent mirror for receiving the laser beam and reflecting back a wavelength separated laser beam, and a beam splitter for dividing the wavelength separated laser beam into a feedback beam directed toward the semiconductor amplifier and an output beam to be coupled out of the optical resonator of the laser source, preferably into an optical fiber.
According to a first aspect of the invention, the laser gain medium comprises a second facet that is partly reflective, so that the second facet emits a second output beam of the laser source, which is preferably coupled into a second optical fiber. The second output beam provides a significantly higher output power than the beam 67 as depicted in FIG. 1, since it is coupled out directly from the gain medium. Additionally, it has been shown that the second output beam provides an improved signal-to-noise ratio with respect to the beam 67, in particular when the gain medium comes to a saturation condition/state. Further more, the second output beam is only influenced by the second facet and not by any other component such as the beam splitter or the like. This leads to an improved radiating characteristic, in particular for the radiation angle and diameter, and allows avoiding interference effects. The second output thus allows a simpler adjustment of a high power beam due to the overall stability of that output, in particular with respect to the beam 67.
The laser system according to the first aspect of the invention allows to execute measurements requiring, at the same time, laser signals with a high signal-to-noise ratio, e.g. for DWDM components, and high power laser signals, e.g. for measuring (saturation effects on Erbium Doped Fiber Amplifiers (EDFA).
According to a second aspect of the invention, the laser source further comprises a mirror arranged in a way that a beam diffracted by the wavelength dependent mirror is reflected back and again diffracted by the wavelength dependent mirror, so that the twice diffracted beam provides the wavelength separated laser beam. Preferably, the mirror is arranged in a Littman-configuration. The additional mirror leads to an improved signal-to-noise ratio and wavelength selectivity, as well for the first as for the second output beam (at the second facet). In particular for the output beam 80, the signal-to-noise ratio can be significantly increased with respect to embodiment as shown in FIG. 1. Furthermore, the additional mirror, in particular when applied in a Littman-configuration, allows providing a continuous and mode-hop free wavelength tuning of the laser source. This is in particular necessary for characterizing optical fiber gratings without requiring high sophisticated measurement setups. Thus, the invention provides a low cost but high quality measurement with respect to solutions requiring expensive wavelength separation or filtering setups.
It is to be understood that, though the first and second aspects of the invention each leads to a significant improvement of the laser system of FIG. 1, both aspects in combination dramatically improve the performance as well of the first as of the second output beam. The first aspect improves the usability of the laser source and renders it possible to provide a wide power range for a plurality of different applications. The second aspect in addition to the first aspect significantly improves the signal-to-noise ratios of the first and second output beams, so that the laser source provides laser signals in a wide power range with improved signal-to-noise ratio.
In a preferred embodiment, the output beam(s) of the laser source is/are preferably coupled into optical fibers, preferably by means of an optical lens and a fixture to align the optical fiber to the optical lens in dependence of the output beam, An optical isolator can be additionally employed for avoiding disturbances of the semiconductor amplifier from any signal outside of the laser source.
The laser source according to the invention is preferably applied in an apparatus for measuring an optical device. The laser source is coupled to and controlled by a wavemeter. The optical device receives an output signal from the laser source and one or more output signals thereof are coupled to one or more power meters. This apparatus provides a low cost measuring setup that allows determining high transmission dynamics characteristics. To enable fast and high accurate measurements, all the components are preferably controlled by one electrical hardware and software structure (e.g., firmware as well as application software, sweep operation). The measurement system provides high reliable and accurate measurement results, shorter measurement times, and is easy to use in particular in an integrated system.